Lumber drying apparatus

ABSTRACT

A lumber drying apparatus for drying lumber such as boards and squares to a desired final moisture content during the progressive transfer of the lumber through a drying section defined by an outer wall structure having an inlet at the green end and outlet at the dry end. The drying section is divided along the lumber transfer path into a plurality of drying compartments, with the atmospheric conditions of the drying compartments individually set according to a drying schedule predetermined by the kind and thickness of the lumber to be dried.

United States Patent Sabuzawa et a1.

Sept. 2, 1975 LUlVmER DRYING APPARATUS Inventors: Takeshi Sabuzawa;Toshiyuki Suzuki; Masayuki Kato; Teruji Muraki, all of Hamamatsu, JapanAssignee:

Filed:

Nippon Gakki Seizo Kabushilki Kaisha, Hamamatsu, Japan Jan. 16, 1974Appl. No.: 433,888

Foreign Application Priority Data Jan. 17, 1973 Japan 48-7536 Jan. 17,1973 Japan 48-7537 Jan. 17, 1973 Japan 48-8107 Jan. 19, 1973 Japan.....48-8771 Jan. 19, 1973 Japan..... 48-8772 Jan. 19, 1973 Japan.....48-8773 Feb. 6, 1973 Japan 48-9376 Feb. 6, 1973 Japan 48-15276 US. Cl.34/1; 34/77; 34/212;

34/219; 34/242 Int. Cl F261) 3/34 Field of Search 34/215, 216, 217, 210,

34/212,1, 77, 78, DIG. 19, DIG. 21, 218, 219, 242; 432/250;'160/l84;432/8, 54; 55/32 [56] References Cited UNITED STATES PATENTS 942,15012/1909 Tiemann 34/77 1,567,031 12/1925 Buensod 34/216 2,197,776 4/1940Argabrite et a1... 34/216 2,799,096 7/1957 Scott 34/216 2,997,096 8/1961Morrison et a1. 34/216 3,474,544 10/1969 Holden, Jr. et a1. 34/13,497,965 3/ 1970 Cortellessa 34/242 3,634,998 1/1972 Patterson 55/32Primary Examiner-Carroll B. Dority, Jr. Assistant Examiner-Larry I.Schwartz Attorney, Agent, or FirmWaters, Schwartz & Nissen [5 7]ABSTRACT A lumber drying apparatus for drying lumber such as boards andsquares to a desired final moisture content during the progressivetransfer of the lumber through a drying section defined by .an outerwall structure having an inlet at the green end and outlet at the dryend. The drying section is divided along the lumber transfer path into aplurality of drying compartments, with the atmospheric conditions of thedrying compartments individually set according to a drying schedulepredetermined by the kind and thickness of the lumber to be dried.

23 Claims, 13 Drawing Figures 13 30 III 1 =12. 39 3 T- 8273?; l8 z I 2,

PATENTED SEP 2 I975 SHEET 3 0F 5 I G. 6b

FIG. 6a

PATENTED 21975 3,902,253 sum u, or 5 FIG. 7

FIG.

31 32 32a 32u /32Q PATENTED SEP 21975 sum 5 u; 5

om mm wN mp muw Um @323 m mm ow 33 mm LUMBER DRYING APPARATUS BACKGROUNDOF THE INVENTION This invention relates to apparatus for drying lumbersuch as boards and squares and, more particularly, to a lumber dryingapparatus, in which the lumber is passed through a plurality of dryingcomparments under different drying conditions preset according to apredetermined schedule to thereby reduce the moisture content of thelumber to a desired value.

The lumber such as boards and squares produced from logs is usuallydried either by air seasoning or through an artificial drying systemsuch as one using hot air.

In the case of air seasoning or natural drying, where the lumber is leftoutdoors in the form of piles or stacks, a large stack area or dryingyard is required. Also, in this case the drying or seasoning periodcannot be controlled and, it is very long; for 2 or 3 years is requiredto season lumber of some kinds. Further, with whatever long seasoningperiod it is impossible to obtain a moisture content lower than theatmospheric equiliblium moisture content.

From this point of view almost all air seasoned lumber flnds use only asconstruction material and cannot be used as the material for interiorfurnishings, furni ture or musical instruments such as a piano wheredimensional error in the material are unacceptable.

Accordingly, the controlled drying, particularly heated drying with hotair, has been adopted to readily obtain lumber suited for the materialof interior furnishings, furniture and musical instruments.

The heated drying method, in contrast to natural drying, permitsreducing the moisture content to a desired value suited to the use ofthe lumber in a greatly reduced drying period.

In this method, hot air whose moisture and temperature are artificiallycontrolled is supplied to a drying compartment where the lumber to betreated is accommodated. From the standpoint of cost and finish ofdrying treatment. however, it to be usual for the lumber to be treatedis preliminarily dried through natural drying. This means that there isstill required a period of several weeks to several months to obtain drylumber of satisfactory finish, that is, one with a moisture content(usually around 12 percent) suited for of furniture and musicalinstruments.

Therefore, a considerable number of drying compartments are needed tomeet the demand for the dry lumber in the market. Also, a large stackarea is still needed for the preliminary drying. Further, suchlaborconsuming operations as piling up the wet lumber in the drying yardand transferring lumber into and out of the drying compartment areinevitable.

SUMMARY OF THE INVENTION A primary object of the invention, accordingly,is to provide a lumber drying apparatus, which permits green lumber suchas boards and squares to be dried during progressive passage thereofthrough a plurality of drying compartments individually held underdifferent temperature and moisture conditions controlled according to adrying schedule predetermined by such factors as the kind and thicknessof the treated lumber, whereby dry lumber with a desired moisturecontent and free from such undesired results as cupping, bowing endsplit, surface checks, inner checks and collapse may be obtained in avery short period of time of such order as expressed in terms of hours.

Another object of the invention is to widely reduce the drying period soas to meet the demand for dry lumber in the market, reduce the spacerequired for drying and facilitate the management of lumber.

A further object of the invention is to obtain dry lumber at a greatlyincreased rate by providing a plurality of vertically spaced lumbertransfer paths in the individual drying compartments.

Still another object of the invention is to improve the efficiency ofdrying and obtain uniform drying of lumber by increasing the innerdiffusion of moisture with at least one high-frequency dielectric dryingunit provided in the path of progressive transfer of the lumber througha succession of drying compartments.

Another object of the invention is to provide a lumber drying apparatus,in which the hot air in each drying compartment is partly recirculatedthrough a duct partly exposed to the external atmosphere and providedwith a moisture removing mechanism to circulate only dry hot air so asto eventually reduce the loss of thermal energy.

Another object of the invention is to provide a lumber drying apparatus,in which preparatory sections are provided adjacent the green end anddry end of the drying section, with a desired temperature gradientbetween the external atmospheric temperature and the temperature of thedrying section established in each preparatory section, therebypreventing bowing, end split and like undesired effects that mightresult from sudden change of temperature and moisture content of thelumber.

Another object of the invention is to provide a lumber drying apparatus,in which each drying compartment is provided with heaters disposed onopposite sides of the lumber transfer path, deflecting members disposedto surround the outer side of the heaters and fans for hot aircirculation, one fan provided on a central portion of the deflectingmember on one side of the lumber transfer path and the other fans onopposite end portions of the deflecting member on the other side,thereby producing two hot air streams flowing in opposite directionswithin the drying compartment so as to dry the lumber uniformly andprevent bowing, end split and like undesired results.

Another object of the invention is to greatly improve the dryingefficiency with the same space factor by providing a plurality ofvertically spaced lumber transfer paths.

Another object is to provide a lumber drying apparatus, in which theindividual drying compartments are vertically divided into a pluralityof divisions by hori- BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is adiagrammatic view outlining the principles of operation of the lumberdrying apparatus according to the invention.

FIGS. 2a and 2b illustrate is a horizontal longitudinal sectional view,partly broken away, showing an embodiment of the lumber drying apparatusaccording to the invention, with a green end portion of the apparatusindicated at FIG. 2a and a dry end portion of the apparatus indicated atFIG. 212.

FIG. 3 is a sectional view taken along line IIIIII in FIG. 2a.

FIG. 4 is a vertical sectional view showing an embodiment of the curtainand high-frequency dielectric drying means provided between adjacentdrying compartments.

FIG. 5 is a vertical sectional view showing an embodiment of the curtainprovided in the inlet preparatory section in the apparatus according tothe invention.

FIGS. 60 and 6b show an embodiment of the means for supplying lumberinto the apparatus according to the invention,

FIG. 7 is a front view showing one of two hot air deflectorsconstituting the cover structure.

FIG. 8 is a sectional view, on an enlarged scale, showing an embodimentof the moisture removing means.

' FIGS. 9a and 9b are views similar to FIGS. 2a and 212 but showing adifferent embodiment of the lumber drying apparatus according to theinvention.

FIG. 10 is a sectional view taken along line XX in FIG. 9a.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows, in block form,the lumber drying apparatus according to the invention. Referencenumeral 1 designates an inlet preparatory section provided at the greenend of the apparatus. Lumber 2 such as boards and squares produced fromlogs are directly fed into the inlet section 1. The interior of theinlet section 1 is held at a temperature of 40C to 50C. Numeral 3designates an outlet preparatory section provided at the dry end of theapparatus. The dried lumber 2' with a moisture content reduced to adesired value is discharged from this section. Similar to the inletsection 1, the outlet section 3 is held at a temperature of 40C to 50C.A temperature gradient is provided between these sections on the onehand and drying compartments on the other hand as will be describedlater. The lumber 2 is transferred from the inlet section 1 to theoutlet section 3 progressively through a succession of a plurality ofdrying compartments, for instance five compartments 4a to 4e, which arerespectively held at different temperature and moisture conditions sothat the lumber with an initial moisture content of nearly 100 percentmay be dried stepwise until the moisture content is reduced to around 10percent.

FIG. 2 shows a sectional view of part of the apparatus in FIG. 1. Theouter walls 5 of the apparatus having the drying compartments 4a to 42are made of a material which is free from thermally-caused chemicalchanges and still having a heat retention property, for instance metal,brick or concrete. The interior of the tunnel-like structure constitutedby the outer walls 5 is divided into the drying compartments 4a to 40 bymeans of curtains 6a to 6d which will be described hereinafter indetail.

As shown in FIG. 2, a number of lumber feed rollers 7 each provided witha heat-resistant rubber coating are arranged in a row at desiredintervals, for instance 1 meter, within and over the entire length ofthe inlet preparatory section 1, drying compartments 4a to 46 and outletpreparatory section 3. These feed rollers 7 extend horizontally andtransversally and are rotatably supported at opposite ends between apair of support members 8 and 9. The feed rollers 7 may be provided attheir one end with respective sprocket wheels in mesh with an endlesschain for driving the feed rollers in synchronism with respect to oneanother.

While only a single roller conveyor type horizontal lumber transfer pathmay be provided, the drying efficiency may be multiplied by providing aplurality of such paths vertically spaced from one another, as shown inFIG. 3, with all the vertically spaced horizontal lumber transfer pathsC C extending through individual drying compartments 4a to 42.

The curtains 6a to 6d defining the individual drying compartments 4a to42 each consists of a pair of curtains spaced apart a desired distanceand constructed such that the lumber being transferred can freely passthrough them.

FIG. 4 shows an example of the construction of the curtains 6a to 6d. Asis shown, each of the curtains in pair is constituted by verticallyspaced shield members 14 individually corresponding to respectivehorizontal lumber transfer paths C C and secured to a pair supportmembers 12 and 13 (only support member 12 being shown) and flap members15 individually pivoted to the lower edge of the respective shieldmembers 14 such that they each can open and close the gap between theadjacent shield members 14.

In the embodiment of FIG. 4, in the space between the pair of curtainsconstituting each of the curtains 6a to 6d, there extend rollers 16 eachbelonging to the corresponding one of the horizontal lumber transferpaths C C and rollers 17 co-operating with the associated rollers 16 toclamp the lumber. The rollers 16 and 17 are driven in synchronism withthe afore-mentioned feed rollers 7 each provided with a sprocket wheel10 in mesh with an endless drive chain 11. The rollers 17 may also beprovided for the respective rollers 7, as shown in FIG. 4, so as toinsure feeding of the lumber and to prevent it from warping. Also, thereare provided in the space between the pair of curtains on the outletside of the rollers 16 and 17 electrode pairs for high-frequencydielectric drying each provided for each lumber transfer path andconsisting of electrodes 18a and 1812 vertically spaced apart a desireddistance.

Referring back to FIG. 2, heaters 19 and 20 having a heater fin fordrying the lumber are provided along the opposite sides of the lumbertransfer path or paths in each of the drying compartments 4a to 4e. Theouter side of each of the heaters 19 and 20 is covered by a coverstructure consisting of hot air deflecting members 21 and 22. Of thepair of cover structures for each drying compartment, one is providedwith suction fans 23 and 24 located in the vicinity of the opposite endsof the compartment, and the other cover structure is provided with asuction fan 25 located to face the center of the compartment. These fans23 to 25 serve to provide circulation of hot air, and they areindependently driven from a motor 26 as shown in FIG. 3. The hot airdeflecting members 21 are so arranged as to cause the air sucked by thefans 23 and 24 to flow along the members 21 and to circulate toward thesuction fan 25, and the hot air deflecting member 22 is so arranged asto cause the air sucked by the fan 25 to flow along the member 22 and tocirculate toward the fans 23 and 24. Numerals 27 and 28 designatenozzles, from which hot air, cool air or steam may be issued to adjustthe atmospheric condition of the drying compartment. These nozzles 27and 28 are provided in the path of hot air circuh tion caused by thefans 23 to 25, that is, on the inner side of the associated coverstructure of the hot air deflecting members 21 and 22.

The cover structure carrying the fans 23 and 24 is provided withventilation ports 29, as is typically shown in FIG. 7, communicatingwith a space 30 defined be tween the cover structure and the outer wall5. The space 30 communicates with the suction side of the fan 25provided in the other cover structure through a duct 31, which isprovided with moisture removing means 32, as is most clearly shown inFIG. 3. The hot air in the drying compartment, is thus partly ledthrough the ports 29 into the space 30 and relieved of moisture in itsflow through the moisture removing means 32 in the duct 31 back to thedrying compartment. The moisture removing means 32 is provided in amajor portion of the duct 31 extending above the outer wall 5.

FIG. 8 shows an embodiment of the moisture removing means 32. Itcomprises a number of vertical fins 32a vertically depending from thetop wall of the duct 31 and slanted fins 32b arranged between adjacentvertical fins 32a and extending from the bottom wall of the duct 31 inan inclined direction such as to redirect the hot air stream. These fins32a and 32b are naturally cooled by the external atmosphere.

The other cover structure carrying the fan 25 is also provided withventilation ports 33 communicating with spaces 34 and 35 defined betweenthe cover structure and outer wall 5, the spaces 34 and 35 communicatingwith the suction side of the fans 23 and 24 through re spective ducts 36and 37, which are provided with respective moisture removing means 38and 39 similar to the one as mentioned above.

The inlet preparatory section 1 is provided with a plurality of curtains40 arranged in a row in the direction of travel of the lumber, and theoutlet preparatory section 3 is also provided with a similar arrangementof curtains 41. The curtains 40 have the same construction as thecurtains 41, as typically shown in FIG. 5. As is shown, each curtaincomprises vertically spaced shield members individually corresponding torespective horizontal lumber transfer paths C C and secured at theopposite ends to a pair support members 42, and flap members 44 of sucha material as heatresistant rubber are individually pivoted to the loweredge of the respective shield members 43 such that they each can openand close the gap between the adjacent shield members 43. These curtainarrangements are provided for establishing a desired temperaturegradient between atmospheric temperature and the temperature of thedrying compartment 4a or 4e, whereby adverse effects of a sudden changeof temperature and moisture conditions on the lumber may be prevented.

The operation of the apparatus of the above construction according tothe invention will now be described.

The heaters 19 and 20 and air blown from the nozzles 27 and 28 arecontrolled according to the drying schedule predetermined by the kindand thickness of the lumber to be dried. In each drying compartment, airheated by the heater 20 and hot air blown from the nozzles 28 tend toproceed across the lumber transfer path toward the fans 22 and 23 due tothe suction effect thereof. Also, air heated by the heater 19 and hotair blown from the nozzles 27 tend to proceed toward the opposite sideof the compartment clue to the suction ef feet of the fan 25 facing thecenter of the compartment. Thus, with the fans 23 to 25 operating, aflow of hot air is provided within the individual drying compartments 4aand 46 in the manner as shown by the arrows in FIG. 1, with the airflowing toward the fan 25 being divided by the deflecting member 22 intobranch streams flowing in opposite directions.

Since the arrangement of the fans and deflecting members is reversed foralternate drying compartments, the direction of flow of hot air is alsoreversed for alternate drying compartments.

The speed of feed of the lumber, that is, the speed of rotation of thefeed rollers 7 constituting the lumber transfer paths C C is alsocontrolled according to the kind and thickness of the lumber. The lumber2 is introduced through the curtains 40 in the inlet preparatory section1 into the first drying compartment 4a, and by the time it leaves thefirst drying compartment 4a, it is dried to a predetermined moisturecontent clue to the corresponding temperature and moisture of the atmosphere in the first drying compartment 4a. Similarly, the moisturecontent of the lumber is progressively reduced as it passes through theatmospheres in the following drying compartments 4b to 4e after thepredetermined drying schedule. The lumber 2' taken out through theoutlet preparatory section 3 following the last drying compartment 46thus has a moisture content suited for the interior furnishings,furnitures and musical instruments.

Also, as the lumber passes through a high-frequency electromagneticfield set up between the electrodes 18a and 18b provided betweenadjacent drying compartments, it is internally heated due to thedielectric loss induced, thus increasing the inner vapor pressure andpromoting the inner diffusion, so that fluctuations in the finalmoisture content may be extremely reduced.

FIG. 6a and 6b show an embodiment of means for introducing lumber 2 intothe roller conveyor type hori zontal lumber transfer paths C, C in theinlet preparatory section 1. It has a vertically movable platform 45movable to the levels of the individual horizontal lumber transfer pathsC C and provided with a lumber loading cylinder 46. In operation, lumber2 is conveyed from a lumber mill 47 through a conveyor 48 to theplatform 45, and after a predetermined number of lumber pieces of arealigned on the platform, the platform may be moved by a suitable drivemeans 45 to the level of the intended lumber transfer path and theloading cylinder 46 is operated to push the aligned pieces lumber intothe intended lumber transfer path.

A specific example of the use of the apparatus according to theinvention for drying agathis lumber (with a moisture content of percent)with a thickness of 16 mm will now be given.

The drying schedule is set as is listed in Table 1 below.

Note: Degree Dry bulb temp. -wet bulb temp.

The agathis lumber was progressively transferred through the individualdrying compartments at a transfer speed of 3.6 meters per hour and theretention time in each drying compartment was set to 4 hours. Table 2lists the moisture contents obtained when the lumber leaves theindividual drying compartments.

Table 2 5 Moisture content (7m First drying compartment 4a 65 Seconddrying compartment 4b 40 Third drying compartment 4c 20 Fourth dryingcompartment 4d 12 Fifth drying compartment 4e 8 As is evident from thisexample, it is possible to dry agathis lumber with initial moisturecontent of 95 per- 15 cent into dry lumber with a moisture content of 8percent which is suitable as material for musical instruments or thelike, in about hours.

FIG. 9 and 10 show another embodiment of the lumber drying apparatusaccording to the invention. In these Figures, similar parts to thoseshown in FIGS. 2 and 3 are designated by like reference numerals and arenot particularly described in any further detail. Similar to theprevious embodiment, the outer wall constituting the individual dryingcompartments is made from such material as metal, brick or concrete freefrom thermally caused chemical changes and having a high heat insulationproperty, and the successive drying compartments 4a to 4e are defined bythe curtains 6a to 6d. In this embodiment, the drying compartments 4a to4e are each vertically divided into two divisions by horizontallyextending partition member P, as shown in FIG. 10. The spaces 10 in boththe vertical divisions are connected to common duct 31 provided withmoisture removing means 32. The duct 31 is 5 forked on the side of themoisture removing means 32 opposite the spaces into two branches, onebeing led to the fan 25 in the upper division, and the other branchindicated at 31a being led to the fan 25 in the lower division. Exceptfor the removal of the moisture which is commonly effected both for theupper and lower divisions, the operation is the same as in the previousembodiment.

We claim:

1. A lumber drying apparatus comprising an outer wall structure defininga drying section having at least one lumber transfer path, said outerwall structure preventing leakage of hot air from said drying section,curtain means in said drying section to divide the same along saidlumber transfer path into a plurality of dry- 5 ing compartments,heating means for heating the interior of each said drying compartment,and means for producing circulation of hot air in each said dryingcompartment including a first fan located on one side of the lumbertransfer path substantially at a position in a transverse central planeof the compartment, second and third fans located on the other side ofthe lumber transfer path substantially at positions symmetrical to eachother with respect to said transverse central plane, first deflectingmeans disposed on opposite sides of said first fan for dividing the flowof hot air with respect to said first fan, said first deflecting meansincluding side walls which incline inwardly into the compartment in adirection away from the center thereof towards the second and thirdfans, and second deflect- 5 ing means disposed between said second andthird fans for redirecting the fiow of hot air between said second andthird fans and said first fan such that two circulating paths of airtraveling in opposite directions are produced in each compartment, saidsecond deflecting means including further side walls which inclineinwardly into the compartment in a direction away from the second andthird fans towards the center of the compartment.

2. The lumber drying apparatus according to claim 1, wherein said dryingsection has a plurality of vertically spaced lumber transfer paths.

3. The lumber drying apparatus according to claim 1, which furthercomprises at least one high-frequency dielectric drying means in saidlumber transfer path.

4. The lumber drying apparatus according to claim 1, wherein saidcurtain means dividing the drying section into a plurality of dryingcompartments each comprises a pair of upright support members onopposite sides of the lumber transfer path, a transverse shield membersecured to said support members, and a flap member made ofheat-resistant material and pivoted to the lower edge of said transverseshield member.

5. The lumber drying apparatus according to claim 1, comprising ductmeans for recirculation of part of the hot air for each of the dryingcompartments, said duct means partly extending to the outside of theouter wall structure and exposed to external atmosphere, and moistureremoving means in the exposed portion of said duct means.

6. The lumber drying apparatus according to claim 1, wherein said outerwall structure further defines an inlet preparatory section adjacent theinlet end of the drying section, said inlet preparatory section beingdivided into a plurality of divisions by curtain means arranged oneafter another in the direction of transfer of lumber, such that atemperature gradient between external atmospheric temperature and thetemperature of the drying section is established in said inletpreparatory section.

7. The lumber drying apparatus according to claim 1, wherein said outerwall structure further defines an outlet preparatory section adjacentthe outlet end of the drying section, said outlet preparatory sectionbeing divided into a plurality of divisions by curtain means arrangedone after another in the direction of transfer of lumber, such that atemperature gradient between the temperature of the drying section andexternal atmospheric temperature is established in said outletpreparatory section.

8. The lumber drying apparatus according to claim 5, wherein saidmoisture removing means comprises a number of vertical fins verticallydepending from the top wall of the duct means and slanted fins betweenadjacent vertical fins and extending from the bottom wall of the ductmeans in an inclined direction such as to redirect the hot air stream.

9. The lumber drying apparatus according to claim 6, wherein saidcurtain means dividing the inlet preparatory section into a plurality ofdivisions each comprises a pair of upright support members on oppositesides of the lumber transfer path, a tranverse shield member secured tosaid support members, and a flap mmember made of heat-resistant materialpivoted to the lower edge of said tranverse shield member.

10. The lumber drying apparatus according to claim 7, wherein saidcurtain means dividing the outlet preparatory section into a pluralityof divisions each comprises a pair of upright support members onopposite sides of the lumber transfer path, a tranverse shield membersecured to said support members, and a flap member made ofheat-resistant material pivoted to the lower edge of said transverseshield member.

11. The lumber drying apparatus according to claim 1, wherein each saiddrying compartment is provided with moist air supply nozzles.

12. The lumber drying apparatus according to claim 1, wherein saidfirst, second and third fans are suction fans. I

13. The lumber drying apparatus according to claim 12, wherein saidfirst deflecting means are disposed on opposite sides of said first fanand adapted to divides a stream of hot air proceeding toward said firstfan into two branch streams flowing in opposite directions toward therespective second and third fans, and said second deflecting meansbetween said second and third fans redirects hot air proceeding fromsaid second and third fans toward said first fan, thereby producing thecirculation of hot air.

14. The lumber drying apparatus according to claim 1, wherein each saiddrying compartment further com prises heater means disposed in the pathof convection of hot air along said first and second deflecting means.

15. The lumber drying apparatus according to claim 1, wherein saidsecond deflecting means in each said drying compartment defines a spacewith respect to the outer wall structure and is provided with aventilating port communicating with said space, and also wherein ductmeans for recirculating part of the hot air is provided for each saiddrying compartment, said duct means partly extending to the outside ofthe outer wall structure and exposed to external atmosphere, whereby thehot air in each drying compartment is partly recirculated through saidventilating port, said space and said duct means to said first fan.

16. The lumber drying apparatus according to claim 15, wherein said ductmeans includes in its portion exposed to external atmosphere with amoisture removing means constituted by a number of vertical finsvertically depending from the top wall of the duct means and slantedfins between adjacent vertical fins and extending from the bottom wallof the duct means in an inclined direction such as to redirect the hotair stream.

17. The lumber drying apparatus according to claim 2, wherein each saidlumber transfer path is constituted by a number of feed rollers arrangedin a row.

18. The lumber drying apparatus according to claim 12, wherein saidsecond deflecting means in each said drying compartment defines a spacewith repsect to the outer wall structure and is provided with aventilating port communicating with said space, and also wherein ductmeans for recirculating part of the hot air is provided for each saiddrying compartment, said duct means partly extending to the outside ofthe outer wall structure and exposed to external atmosphere, whereby thehot air in each drying compartment is partly recirculated through saidventilating port, said space and said duct means to said first fan.

19. The lumber drying apparatus according to claim 18, wherein said ductmeans includes in its portion exposed to external atmosphere a moistureremoving means constituted by a number of vertical fins verticallydepending from the top wall of the duct means and slanted fins betweenadjacent vertical fins and extending from the bottom wall of the ductmeans in an inclined direction such as to redirect the hot air stream.

20. The lumber drying apparatus according to claim 1 wherein said firstdeflecting means extends along said one side of the associatedcompartment and said second deflecting means extends along the otherside of the associated compartment.

21. The lumber drying apparatus according to claim 20 wherein said sidewalls of the first deflecting means have end portions which graduallyslope inwardly into the compartment in a direction away from said firstfan.

22. The lumber drying apparatus according to claim 4 wherein said firstfan and said second and third fans are on alternately opposite sides ofadjacent compartments.

23. The lumber drying apparatus according to claim 22 wherein the sidewalls of the first deflecting means of one compartment join the sidewalls of the second deflecting means of an adjacent compartment at thejuncture of these compartments.

l l l

1. A lumber drying apparatus comprising an outer wall structure defininga drying section having at least one lumber transfer path, said outerwall structure preventing leakage of hot air from said drying section,curtain means in said drying section to divide the same along saidlumber transfer path into a plurality of drying compartments, heatingmeans for heating the interior of each said drying compartment, andmeans for producing circulation of hot air in each said dryingcompartment including a first fan located on one side of the lumbertransfer path substantially at a position in a transverse central planeof the compartment, second and third fans located on the other side ofthe lumber transfer path substantially at positions symmetrical to eachother with respect to said transverse central plane, first deflectingmeans disposed on opposite sides of said first fan for dividing the flowof hot air with respect to said first fan, said first deflecting meansincluding side walls which incline inwardly into the compartment in adirection away from the center thereof towards the second and thirdfans, and second deflecting means disposed between said second and thirdfans for redirecting the flow of hot air between said second and thirdfans and said first fan such that two circulating paths of air travelingin opposite directions are produced in each compartment, said seconddeflecting means including further side walls which incline inwardlyinto the compartment in a direction away from the second and third fanstowards the center of the compartment.
 2. The lumber drying apparatusaccording to claim 1, wherein said drying section has a plurality ofvertically spaced lumber transfer paths.
 3. The lumber drying apparatusaccording to claim 1, which further comprises at least onehigh-frequency dielectric drying means in said lumber transfer path. 4.The lumber drying apparatus according to claim 1, wherein said curtainmeans dividing the drying section into a plurality of dryingcompartments each comprises a pair of upright support members onopposite sides of the lumber transfer path, a transverse shield membersecured to said support members, and a flap member made ofheat-resistant material and pivoted to the lower edge of said transverseshield member.
 5. The lumber drying apparatus according to claim 1,comprising duct means for recirculation of part of the hot air for eachof the drying compartments, said duct means partly extending to theoutside of the outer wall structure and exposed to external atmosphere,and moisture removing means in the exposed portion of said duct means.6. The lumber drying apparatus according to claim 1, wherein said outerwall structure further defines an inlet preparatory section adjacent theinlet end of the drying section, said inlet preparatory section beingdivided into a plurality of divisions by curtain means arranged oneafter another in the direction of transfer of lumber, such that atemperature gradient between external atmospheric temperature and thetemperature of the drying section is established in said inletpreparatory section.
 7. The lumber drying apparatus according to claim1, wherein said outer wall structure further defines an outletpreparatory section adjacent the outlet end of the drying section, saidoutlet preparatory section being divided into a plurality of divisionsby curtain means arranged one after another in the direction of transferof lumber, such that a temperature gradient between the temperaturE ofthe drying section and external atmospheric temperature is establishedin said outlet preparatory section.
 8. The lumber drying apparatusaccording to claim 5, wherein said moisture removing means comprises anumber of vertical fins vertically depending from the top wall of theduct means and slanted fins between adjacent vertical fins and extendingfrom the bottom wall of the duct means in an inclined direction such asto redirect the hot air stream.
 9. The lumber drying apparatus accordingto claim 6, wherein said curtain means dividing the inlet preparatorysection into a plurality of divisions each comprises a pair of uprightsupport members on opposite sides of the lumber transfer path, atranverse shield member secured to said support members, and a flapmmember made of heat-resistant material pivoted to the lower edge ofsaid tranverse shield member.
 10. The lumber drying apparatus accordingto claim 7, wherein said curtain means dividing the outlet preparatorysection into a plurality of divisions each comprises a pair of uprightsupport members on opposite sides of the lumber transfer path, atranverse shield member secured to said support members, and a flapmember made of heat-resistant material pivoted to the lower edge of saidtransverse shield member.
 11. The lumber drying apparatus according toclaim 1, wherein each said drying compartment is provided with moist airsupply nozzles.
 12. The lumber drying apparatus according to claim 1,wherein said first, second and third fans are suction fans.
 13. Thelumber drying apparatus according to claim 12, wherein said firstdeflecting means are disposed on opposite sides of said first fan andadapted to divides a stream of hot air proceeding toward said first faninto two branch streams flowing in opposite directions toward therespective second and third fans, and said second deflecting meansbetween said second and third fans redirects hot air proceeding fromsaid second and third fans toward said first fan, thereby producing thecirculation of hot air.
 14. The lumber drying apparatus according toclaim 1, wherein each said drying compartment further comprises heatermeans disposed in the path of convection of hot air along said first andsecond deflecting means.
 15. The lumber drying apparatus according toclaim 1, wherein said second deflecting means in each said dryingcompartment defines a space with respect to the outer wall structure andis provided with a ventilating port communicating with said space, andalso wherein duct means for recirculating part of the hot air isprovided for each said drying compartment, said duct means partlyextending to the outside of the outer wall structure and exposed toexternal atmosphere, whereby the hot air in each drying compartment ispartly recirculated through said ventilating port, said space and saidduct means to said first fan.
 16. The lumber drying apparatus accordingto claim 15, wherein said duct means includes in its portion exposed toexternal atmosphere with a moisture removing means constituted by anumber of vertical fins vertically depending from the top wall of theduct means and slanted fins between adjacent vertical fins and extendingfrom the bottom wall of the duct means in an inclined direction such asto redirect the hot air stream.
 17. The lumber drying apparatusaccording to claim 2, wherein each said lumber transfer path isconstituted by a number of feed rollers arranged in a row.
 18. Thelumber drying apparatus according to claim 12, wherein said seconddeflecting means in each said drying compartment defines a space withrepsect to the outer wall structure and is provided with a ventilatingport communicating with said space, and also wherein duct means forrecirculating part of the hot air is provided for each said dryingcompartment, said duct means partly extending to the outside of theouter wall structure and exposed to external atmosphere, whereby the hotair in each drying compartment is partly recircUlated through saidventilating port, said space and said duct means to said first fan. 19.The lumber drying apparatus according to claim 18, wherein said ductmeans includes in its portion exposed to external atmosphere a moistureremoving means constituted by a number of vertical fins verticallydepending from the top wall of the duct means and slanted fins betweenadjacent vertical fins and extending from the bottom wall of the ductmeans in an inclined direction such as to redirect the hot air stream.20. The lumber drying apparatus according to claim 1 wherein said firstdeflecting means extends along said one side of the associatedcompartment and said second deflecting means extends along the otherside of the associated compartment.
 21. The lumber drying apparatusaccording to claim 20 wherein said side walls of the first deflectingmeans have end portions which gradually slope inwardly into thecompartment in a direction away from said first fan.
 22. The lumberdrying apparatus according to claim 4 wherein said first fan and saidsecond and third fans are on alternately opposite sides of adjacentcompartments.
 23. The lumber drying apparatus according to claim 22wherein the side walls of the first deflecting means of one compartmentjoin the side walls of the second deflecting means of an adjacentcompartment at the juncture of these compartments.